In the nuclear energy industry, a large capital expenditure is required to construct plant facilities. In order to increase the return on this expenditure, it is desirable to operate existing plants at higher power output levels. However, increase in output results in an accompanying increase in neutron flux (fluence) to the reactor vessel (RV) which contributes to embrittlement of RV material.
Such embrittlement increases the hazards posed by pressurized thermal shock (PTS) events. These events are system transients in a pressurized water reactor (PWR) that can cause severe overcooling followed by rapid repressurization. A particularly severe PTS event coupled with a highly embrittled material in the RV beltline could cause a pre-existing flaw to propagate through the vessel wall.
Accordingly, PTS criteria have been mandated which require RT.sub.PTS values of 270.degree. F. for plates, forgings and axial weld materials, and of 300.degree. F. for circumferential weld materials used in RVs. These values are to be compared to RT.sub.PTS values calculated for each of the beltline materials based on a prescribed margin which accounts for uncertainties and a predicted increase (shift) in the reference temperature. The shift is computed using the best estimate of the copper and nickel content of the material and of the fast neutron fluence for the period of service in question. Thus, it is advantageous to minimize the fast neutron fluence.
According to existing designs, core shroud panels, in conjunction with the core support barrel (CSB), and thermal shield limit neutron fluence to the RV materials. The core shroud assemblies have been designed and constructed in view of structural conditions affecting the various RV structural components, particularly CSB assembly. These conditions include structural integrity, thermal conductivity, hydraulic load level, flow induced vibration, corrosion resistance, thermal loads, thermal shield preload, coolant flow, alignment, distortion, vibration, weight, and stress. Limits for these conditions are established, either directly or indirectly, under current regulations.
While it has been attempted to reduce the fluence to the reactor vessel by introducing extra material outside of the core support barrel, such attempts suffer from several drawbacks. In particular, such designs suffer from placement of extra material in a flow environment in which a severe level of flow induced vibrations is created.
Consequently, there is a need to limit the neutron flux reaching the RV, particularly at critical areas, in a manner which is consistent with current and anticipated design guidelines. In particular, this limiting of the fluence must not substantially interfere with water flow in the vessel and be subject to severe flow induced vibrations. Apart from permitting greater power output, fulfilling this need would permit maintenance of low levels of neutron fluence over extended periods of time at current power output levels, relaxation of current fuel management schemes, and maintenance of an extra margin of security for continuing plant operation in the event the PTS screening criteria becomes more burdensome.